Oxidation method

ABSTRACT

A process for oxidizing a starting material with an oxidizing agent to obtain a product, which comprises carrying out the oxidation in a reaction apparatus which has a bottom region at the lower end, a top region at the upper end and a reaction zone between the top region and the bottom region, maintaining the reaction mixture in the boiling state in the reaction zone, and introducing oxidizing agent into the reaction zone in at least two substreams.

The present invention relates to a process for oxidizing a startingmaterial with an oxidizing agent to obtain a product,

-   -   which comprises    -   carrying out the oxidation in a reaction apparatus which has    -   a bottom region at the lower end,    -   a top region at the upper end and    -   a reaction zone between the top region and the bottom region,    -   maintaining the reaction mixture in the boiling state in the        reaction zone, and    -   introducing oxidizing agent into the reaction zone in at least        two substreams.

Numerous processes for oxidizing a starting material, in particular anorganic starting material, with a molecular oxygen-containing gas toobtain a product are known.

For example, saturated compounds may be converted to unsaturatedcompounds, such as methylcyclohexane to toluene or propane to propene,alcohols to aldehydes or ketones, such as isopropanol to acetone,s-butanol to methyl ethyl ketone or methanol to formaldehyde,hydrocarbons to hydroperoxides, such as cumene to cumene hydroperoxide,tetralin to tetralin hydroperoxide or cyclohexane to cyclohexanehydroperoxide, olefins to epoxides, such as ethene to ethylene oxide, orhydrocarbons to alcohols, aldehydes, ketones or carboxylic acids, suchas cyclohexane to cyclohexanol or cyclohexanone, toluene to benzaldehydeor benzoic acid, o-, m- or p-xylene to the corresponding aromaticdicarboxylic acids or their anhydrides, butane to maleic anhydride orpropene to acrolein or acrylic acid.

One of the problems of such oxidation reactions is that the desiredproducts of value may themselves likewise be oxidized to obtainundesired by-products or ultimately carbon dioxide and water. Thisdisadvantageously leads to a reduction in the selectivity of theoxidation reaction.

An industrially significant oxidation is described in: Weissermel/Arpe,Industrielle Organische Chemie, 4th edition, VCH, Weinheim, 1994, pages260 ff and is the oxidation of cyclohexane to a mixture comprisingcyclohexanol and cyclohexanone in the liquid phase using air in thepresence of manganese or cobalt salts as catalyst at 125-165° C. and apressure in the range from 8 to 15 bar (absolute).

In this oxidation, the cyclohexane conversion is limited in order toachieve an industrially viable selectivity. According to Arpentier etal., The Technology of Catalytic Oxidations, Editions Technip 2001,pages 226 ff, the selectivity in cyclohexane conversions in the range of1-2% is approx. 90%, while even at conversions of 4-5% it falls to77-85%.

The unconverted cyclohexane has to be distilled off in a downstreamdistillation column and recycled into the oxidation stage.

Cyclohexanol and cyclohexanone are starting materials for preparingcaprolactam and adipic acid which are both in turn used to aconsiderable extent as monomers for preparing industrially significantpolyamides.

DE 19811517 describes the uncatalyzed, selective oxidation ofcyclohexane with ozone to cyclohexanone in a reactor inertized towardozone by metering the ozone in via the top of the column, while at thesame time continuously removing the cyclohexanone formed at the bottomof the column as the product.

A disadvantage of this process is the insufficient contact of theoxidizing agent with the starting material and the poor utilization ofthe oxidizing agent: at industrially relevant pressures, ozone isgaseous and therefore leaves the reactor again without sufficientcontact with the hydrocarbon to be oxidized.

Furthermore, the process is intended to be carried out at temperaturesless than or equal to the boiling temperature of the cyclohexane to beoxidized. However, since the reaction products boil approx. 75° C.higher than the starting material and the boiling temperature of thereaction mixture is therefore above the boiling temperature of thecyclohexane, this process is a pure liquid phase reaction withoutdistillation. This process therefore has the disadvantages alreadymentioned above with regard to the separation of the reaction mixtureand recycling of the cyclohexane.

It is an object of the present invention to provide a process whichfacilitates the oxidation of a starting material, in particular anorganic starting material, with an oxidizing agent to obtain a productin a technically simple and economical manner while avoiding thedisadvantages mentioned.

We have found that this object is achieved by the process defined at theoutset.

According to the invention, the present process is suitable foroxidizing a starting material.

Useful starting materials are inorganic, but preferably organic,compounds.

Useful organic compounds may be unsaturated, but preferably saturated,hydrocarbons. In these hydrocarbons, one or more carbon atoms may bereplaced by heteroatoms, such as oxygen, nitrogen, sulfur or phosphorus,with the saturation of any free valencies of such heteroatoms byhydrogen or substituents, in particular the substituents specifiedhereinbelow for the hydrocarbons; preference is given to no carbon atomsbeing replaced by such heteroatoms. For the purposes of the presentinvention, the hydrocarbons both with and without such heteroatoms arereferred to in summary as hydrocarbons.

Useful unsaturated hydrocarbons include those having one or more triplebonds, one or more olefinic double bonds or aromatic systems, or thosewhich have a combination of such features, such as ethene, propene,1-butene, 2-butene, 1,3-butadiene, benzene, toluene, o-xylene, m-xylene,p-xylene, fluorene, 2-methylpyridine, 3-methylpyridine,4-methylpyridine, and tetralin. Useful unsaturated hydrocarbons may belinear or cyclic.

Useful saturated hydrocarbons may be linear or preferably cyclicalkanes, in particular those having from 2 to 12 carbon atoms.

Advantageous linear alkanes are ethane, propane, n-butane, i-butane,n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane.

Useful cyclic alkanes may be cyclohexane and decalin.

The hydrocarbons may be unsubstituted or substituted, for example byaliphatic groups, preferably C₁-C₈-alkyl groups, such as methyl, ethyl,i-propyl, n-propyl, n-butyl, i-butyl, s-butyl, n-pentyl, n-hexyl,n-heptyl, n-octyl, 2-ethylhexyl, OH, ═O, C₁-C₈-alkoxy, COOH,C₂-C₆-carbalkoxy, C₁-C₁₀-acyloxy or C₁-C₈-alkylamino, sulfonic acid ortheir salts, such as alkali metal or alkaline earth metal salts, oresters, cyano, or halogens such as fluorine, chlorine or bromine.

In an advantageous embodiment, the process according to the inventionmay be applied to the oxidation of hydrocarbons or aldehydes tohydroperoxides which may be used, for example, in the indirectepoxidation of olefins, such as acetaldehyde to peracetic acid,isobutane to isobutyl peroxide, isopentane to isopentyl peroxide,ethylbenzene to phenylethyl peroxide, cumene to cumene hydroperoxide, ortetralin to tetralin hydroperoxide.

In a further advantageous embodiment, the process according to theinvention may be applied to the oxidation of hydrocarbons or aldehydesto acids or their anhydrides or their ester, such as p-xylene toterephthalic acid, m-xylene to isophthalic acid, o-xylene to phthalicacid or phthalic anhydride, n-butane to acetic acid, toluene tobenzaldehyde or benzoic acid, paraffins to acids, acetaldehyde to aceticacid, trimethylbenzene to hemimellitic acid, n-butyraldehyde ton-butyric acid, crotonaldehyde to crotonic acid, butane to ethylacetate, butene to maleic anhydride, butane to maleic anhydride, benzeneto maleic anhydride, or propene to acrylic acid.

In a further advantageous embodiment, the process according to theinvention may be applied to the oxidation of hydrocarbons or aldehydesto ketones, alcohols or quinones, such as fluorene to fluorenone,trimethylphenol to trimethylquinone, acetaldehyde to acetic anhydride,naphthalene to naphthoquinone, anthracene to anthraquinone,p-diisopropylbenzene to hydroquinone, p-methylisopropylbenzene tocresol, or paraffins to alcohols. In a further advantageous embodiment,the process according to the invention may be applied to the oxidationof alcohols to aldehydes or ketones, such as isopropanol to acetone,s-butanol to methyl ethyl ketone, or methanol to formaldehyde.

In a further advantageous embodiment, the process according to theinvention may be applied to the oxidation of C—C single bonds to C—Cmultiple bonds, such as butene to butadiene, ethylbenzene to styrene,methylcyclohexane to toluene, or propane to propene.

In a further advantageous embodiment, the process according to theinvention may be applied to the oxidation of hydrocarbons to nitriles,such as the oxidation of toluene with N₂O to benzonitrile.

In a further preferred embodiment, the process according to theinvention may be applied to the oxidation of C—C single bonds or C—Cmultiple bonds using ozone to obtain an acid function, such as theozonolysis of natural products to fatty acids.

In a further advantageous embodiment, the process according to theinvention may be applied to the oxidation of C—C multiple bonds usinghydrogen peroxide to obtain the corresponding diols, such as allylalcohol to glycerol.

The hydrocarbons may be used as individual compounds or as a mixture ofsuch hydrocarbons.

In a particularly preferred embodiment, the starting material used maybe cyclohexane.

Advantageous products in this case are cyclohexanol, cyclohexanone,cyclohexyl hydroperoxide or their mixtures, in particular cyclohexanol,cyclohexanone or their mixtures.

According to the invention, a starting material is oxidized using anoxidizing agent.

In an advantageous embodiment, the oxidizing agent used may be amolecular oxygen-containing gas, in particular molecular oxygen.

The molecular oxygen used may be dioxygen in the triplet or singletstate or trioxygen, i.e. ozone, preferably dioxygen, in particular inthe triplet state, or mixtures of such molecular forms of oxygen. Thegas comprising such molecular oxygen may be free of further components.

The gas containing such molecular oxygen may comprise further, differentcomponents.

Useful further, different components include oxidizing gases, such asnitrogen oxides.

In the case of further, different components, it may be preferable touse inert gases, i.e. those which do not enter substantially into theoxidation reaction, if at all, in the process according to theinvention, such as nitrogen, for example in the form of air, or noblegases, for example, argon, or their mixtures.

In a further preferred embodiment, the oxidizing agent used may be a gascomprising one or more nitrogen oxides, in particular one or morenitrogen oxides.

Useful nitrogen oxides include dinitrogen monoxide, nitrogen monoxide,nitrogen dioxide, and their mixtures or oligomers. The gas comprisingone or more such nitrogen oxides may be free of further components.

The gas comprising one or more such nitrogen oxides may contain further,different components.

Useful further, different components include oxidizing gases, such asoxygen.

In the case of further, different components, it is advantageous to useinert gases, i.e. those which do not enter substantially into theoxidation reaction, if at all, in the process according to theinvention, such as nitrogen, for example in the form of air, or noblegases, for example argon, or their mixtures.

In a further preferred embodiment, the oxidizing agent used may be acompound which is liquid under the reaction conditions, such as aperoxide, for example an inorganic peroxide, such as hydrogen peroxide,or an organic peroxide, such as cyclohexane hydroperoxide, isobutylhydroperoxide, isopentyl hydroperoxide, phenylethyl hydroperoxide,cumene hydroperoxide, tetralin hydroperoxide, or a peracid, such asperacetic acid.

The mixing ratios between starting material used and the molecularoxygen in the molecular oxygen-containing gas depends on the desireddegree of conversion of the starting material to the product from achemical point of view, for example the conversion of an alkane to analcohol or a ketone, and from a process engineering point of view, i.e.the desired conversion, and may be easily optimized by a few simplepreliminary experiments.

Oxidizing agent and starting material may be added separately to thereaction apparatus.

Oxidizing agent and starting material can be partially mixed beforeaddition to the reaction apparatus and added to the reaction apparatus.

Oxidizing agent and starting material can be completely mixed beforeaddition to the reaction apparatus and added to the reaction apparatus.

According to the invention, the oxidation is carried out in a reactionapparatus which has

-   -   a bottom region at the lower end,    -   a top region at the upper end and    -   a reaction zone between the top region and the bottom region.

Preferred reaction apparatus are rectification columns, as described,for example, in Kirk-Othmer, Encyclopedia of Chemical Technology, 3.Ed., Vol. 7, John Wiley & Sons, New York, 1979, pages 870-881, such astray columns, for example sieve tray columns or bubble-cap tray columns,or columns having structured packings or random packings.

In a preferred embodiment, useful trays are those which facilitate along residence time of the reaction mixture in the column, such as valvetrays, preferably bubble-cap trays or tunnel-cap trays.

In a further preferred embodiment, structured packings, such as wovenmetal packings or sheet metal packings, advantageously having an orderedstructure, or random packings are contemplated.

In a further preferred embodiment, hold-up packings are considered. Suchhold-up packings allow the residence time in the reaction zone to beadjusted with the aid of the pressure drop and, even at high load,ensure a good separation performance.

In a particularly preferred embodiment, it is possible to use internalshaving a high number of plates, such as woven metal packings or sheetmetal packings, advantageously having an ordered structure, below thelowermost feedpoint for the oxidizing agent into the reaction apparatus.

Advantageously, the rectification column should have a separationperformance of from 10 to 100, preferably from 20 to 40, theoreticalplates.

Advantageously, the higher-boiling reactant of the two reactantsstarting material and oxidizing agent may be fed to the reactionapparatus predominantly or completely above the lower-boiling reactant,and in particular, the higher-boiling reactant may be fed into the uppersection of the rectification column and the lower-boiling reactant intothe lower section of the rectification column.

The higher-boiling reactant may comprise lower-boiling reactant.

The lower-boiling reactant may comprise higher-boiling reactant.

In a particularly preferred embodiment, the rectification column has adistillation section between the reaction section and bottom.

It has proven particularly advantageous to install from 0 to 50,preferably from 5 to 30, theoretical plates in the lower section of therectification column, i.e. the distillation section.

It has proven particularly advantageous to install from 0 to 50,preferably from 5 to 30, theoretical plates in the upper section of therectification column, i.e. the reaction zone. The reaction zone may besituated within the rectification section of the column.

The reaction zone may be situated outside the rectification section ofthe column.

The reaction zone may be situated outside the rectification column.

In this case, the pressure in the reaction zone and the pressure in therectification column may be the same or different.

FIG. 1 shows a schematic of an advantageous embodiment of a reactionapparatus. In FIG. 1:

1: reaction section

2: distillation section

3: feed for starting material

4: feed for catalyst

5: addition of oxidizing agent, in particular gaseous oxidizing agent,such as air

6: evaporator

7: product stream

8: heat exchanger

9: discharge of inerts

10: separator

11: water discharge

12: starting material recycling

The process according to the invention may preferably be carried out ina plurality of reaction apparatus connected in series. When operatingthe downstream reaction apparatus at a lower pressure, a portion of theenergy contained in the vapor stream of the upstream column mayadvantageously be transferred to the feed stream of one of thedownstream reaction apparatus.

Furthermore, a portion of the uncondensed vapor stream mayadvantageously be recycled into the lower section of the reactionapparatus. This cycle gas method allows a portion of the energy presentin the bottom stream to be recovered.

The average residence time of the reaction mixture on the trays of thecolumn should be from 1 to 120 minutes, preferably from 5 to 30 minutes.

The process according to the invention, in particular when cyclohexaneis used as the starting material, may preferably be carried out at apressure in the range from 0.1 to 3.5 MPa, preferably from 0.5 to 2.5MPa, measured in the bottom region of the reaction apparatus.

The temperature is then considered with regard to the maintenance in theboiling state of the reaction mixture in the reaction zone. Thetemperature suitable for this purpose for the particular reaction may beeasily determined by a few simple preliminary experiments.

When cyclohexane is used as the starting material, advantageoustemperatures in the reaction zone are in the range from 70 to 220° C.,preferably from 120 to 190° C.

In a further preferred embodiment, the reaction apparatus may have ameans for withdrawing gases at the upper end of the upper section.

Advantageously, the reaction is carried out in such a way that reactionmixture present below the reaction zone is evaporated to obtain amixture of liquid and gaseous reaction mixture.

In an advantageous embodiment, the reaction apparatus is filled withliquid reaction mixture in the bottom region and in the region of thereaction zone.

Owing to its lower density compared to the liquid reaction mixture, thegaseous reaction mixture obtained in this way then rises in thedirection of the top region of the reaction apparatus. Owing to theinteraction between the gaseous and the liquid phase, condensation andevaporation processes may result in changes in the composition of thegas phase.

According to the invention, the gaseous reaction mixture arriving in thetop region of the reaction apparatus is condensed and fed thus to thereaction zone, advantageously in the liquid phase.

According to the invention, the oxidizing agent is introduced into thereaction zone in at least 2, preferably from 2 to 100, in particularfrom 2 to 50, more preferably from 2 to 40, such as 2, 3, 4, 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, substreams.

The oxidizing agent may be introduced into the reaction apparatus byprocesses known per se, in particular for the introduction of a gas intoa liquid.

The process according to the invention may be carried out without acatalyst.

The process according to the invention may be carried out in thepresence of a homogeneous or heterogeneous catalyst.

When a homogeneous catalyst is used, this may advantageously be added tothe reaction mixture in the top region of the reaction apparatus andwithdrawn with the reaction mixture in the bottom region.

When a heterogeneous catalyst is used, this may advantageously beimmobilized in the reaction zone of the reaction apparatus by processesknown per se.

In general, catalysts known per se may be used for the particularoxidation reactions, for example, in the case of the oxidation ofcyclohexane to cyclohexanol, cyclohexanone or its mixtures, cobalt ormanganese salts.

The amounts of catalyst may easily be determined in accordance with thecatalyst velocities known for these catalysts for the particularreactions and the conversions selected in the process according to theinvention, and an optimization of the catalyst amounts may easily becarried out by a few simple preliminary experiments.

Advantageously, a reaction mixture comprising the product may bewithdrawn in the bottom region of the reaction apparatus, in particularwhen the boiling point of the product is higher than the boiling pointof the starting material under the reaction conditions. The reactionmixture withdrawn in the bottom region may consist of product or amixture which comprises the product in addition to further components,such as starting material, by-products and secondary products.

Advantageously, a reaction mixture comprising the product may bewithdrawn in the top region of the reaction apparatus, in particularwhen the boiling point of the product is lower than the boiling point ofthe starting material under the reaction conditions. The reactionmixture withdrawn in the top region may consist of product or a mixturewhich comprises the product in addition to further components, such asstarting material, by-products and secondary products.

When water is to occur in the oxidation reaction according to theinvention as an inevitable or undesired by-product or as a secondaryproduct, this may advantageously be withdrawn during the oxidation fromthe reaction apparatus above the reaction zone, advantageously overhead.

COMPARATIVE EXAMPLE 1

In a bubble column reactor divided into eight chambers, the cyclohexanestream which had been added at the upper end of the reactor was adjustedin such a way that the residence time of the liquid phase in the reactorwas 31 minutes. By adding an appropriate amount of air evenlydistributed over the chambers of the reactor, a cyclohexane conversionof 3.5% was set. The reactor was operated at a pressure of 16 bar.

The total selectivity for cyclohexanol, cyclohexanone and cyclohexanehydroperoxide was 83.9%. The space-time yield, based on the liquid phasein the reactor, was 45.7 kg/(m³*h).

EXAMPLE 1

2415 kg/(m³*h) of cyclohexane, based on the liquid phase volume, werefed above the reaction section to a reaction column having 10 trays inthe reaction section (upper) and 10 trays in the distillation section(lower). The column was operated at a pressure of 11.9 bar. 0.15 m³(STP) of air per kg of cyclohexane was introduced uniformly distributedover the 10 trays of the reaction section of the column. At anevaporator energy based on the fresh cyclohexane stream of 200 Wh/kg,the cyclohexane conversion was 10.1%.

The total selectivity for cyclohexanol, cyclohexanone and cyclohexanehydroperoxide was 88.0%. The space-time yield based on the liquid phasein the reactor was 250 kg/(m³*h).

COMPARATIVE EXAMPLE 2

Example 1 was repeated, with the difference that all of the air wasintroduced in one stream into the lowermost tray of the reactionsection.

The cyclohexane conversion was 9.8%.

The total selectivity for cyclohexanol, cyclohexanone and cyclohexanehydroperoxide was 84.1%. The space-time yield, based on the liquid phasein the reactor, was 232 kg/(m³*h).

1. A process for oxidizing a starting material with an oxidizing agentto obtain a product which comprises carrying out the oxidation in areaction apparatus which has a bottom region at the lower end, a topregion at the upper end and a reaction zone between the top region andthe bottom region, maintaining the reaction mixture in the boiling statein the reaction zone, and introducing oxidizing agent into the reactionzone in at least two substreams.
 2. A process as claimed in claim 1,wherein the unconverted starting material leaving the reaction zone isrecycled into the reaction zone.
 3. A process as claimed in claim 1,wherein the starting material used is a linear or cyclic alkane.
 4. Aprocess as claimed in claim 1, wherein the oxidizing agent used is anoxidizing agent which is gaseous under the reaction conditions.
 5. Aprocess as claimed in claim 4, wherein the oxidizing agent used is amolecular oxygen-containing gas.
 6. A process as claimed in claim 1,wherein the oxidation is carried out in the presence of a catalyst.
 7. Aprocess as claimed in claim 1, wherein water is by-produced in theoxidation and this water is withdrawn during the oxidation from thereaction apparatus in the reaction zone or in the top region.
 8. Aprocess as claimed in claim 1, which is carried out at a temperature inthe range from 10 to 300° C., measured in the reaction zone.
 9. Aprocess as claimed in claim 1, wherein the reaction apparatus used is arectification column.
 10. A process as claimed in claim 1, wherein thestarting material is oxidized with cycle gas which is enriched with anoxidizing agent.
 11. A process as claimed in claim 1, wherein aproduct-containing reaction mixture is withdrawn below the reactionzone.
 12. A process as claimed in claim 1, wherein the higher-boilingreactant selected from the group consisting of oxidizing agent andstarting material is fed to the reaction apparatus above thelower-boiling reactant selected from the group consisting of oxidizingagent and starting material.
 13. A process as claimed in claim 1,wherein the starting material used is cyclohexane.
 14. A process asclaimed in claim 1, wherein cyclohexane is oxidized with air, reactionmixture is continuously withdrawn in the bottom region of the reactionapparatus and unconverted cyclohexane and water are continuously removedin the top region, cyclohexane and water are separated by means of aphase separator and the resulting cyclohexane is fed to the top regionof the reaction apparatus as reflux.
 15. A process as claimed in claim2, wherein the starting material used is a linear or cyclic alkane andthe oxidizing agent used is an oxidizing agent which is gaseous underthe reaction conditions.
 16. A process as claimed in claim 15, whereinthe oxidation is carried out in the presence of a catalyst and water isby-produced in the oxidation and this water is withdrawn during theoxidation from the reaction apparatus in the reaction zone or in the topregion.
 17. A process as claimed in claim 16, which is carried out at atemperature in the range from 10 to 300° C., measured in the reactionzone and the reaction apparatus used is a rectification column.